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DIVISION S-9—SOIL MINERALOGY

Investigation into the Origin of Magnetic Soils on the Oak Ridge Reservation, Tennessee

^a Environmental Sciences Division, Oak Ridge National Lab., P.O. Box 2008, Oak Ridge, TN 37831-6038
^b Jr., Geology Dep., Temple Univ., 1901 N. 13th St., Beury Hall, Philadelphia, PA 19122-6081

^* Corresponding author (rivers{at}geoladm.geol.queensu.ca)

In 1993–1994, researchers at Oak Ridge National Laboratory collected high-resolution airborne geophysical data on the Oak Ridge Reservation, Oak Ridge, TN. The data were collected in part to address concerns about possible undocumented hazardous waste sites. Interpretation of the aeromagnetic data was complicated, however, by the discovery in remote areas of numerous small magnetic anomalies of natural origin. Magnetic susceptibility measurements of core showed that the underlying Copper Ridge Dolomite was non-magnetic. We attribute the magnetic anomalies to the presence of the ferromagnetic mineral maghemite (-Fe₂O₃), which formed during pedogenesis of Fe-enriched colluvium that had infilled low-lying areas, including dolines. We discuss explanations offered in the literature for the formation of magnetic soils, and present evidence based on profile descriptions, thin sections, x-ray diffraction (XRD), and scanning electron microscopy (SEM), that in this case, maghemite formed either by anaerobic microbial Fe reduction followed by the formation of single-domain maghemite, or by abiological weathering and reduction of an Fe-bearing mineral followed by oxidation. Naturally occurring magnetic soils may produce magnetic anomalies similar to those characteristics of anthropogenic objects, such as buried waste drums, and complicate interpretation of airborne geophysical surveys.

Abbreviations: EDX, energy dispersive x-ray analysis • ICPMS, inductively coupled plasma-mass spectrometry • ORNL, Oak Ridge National Laboratory • SEM, scanning electron microscopy • XRD, x-ray diffraction

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